Last point - surrendering, calling a truce or running away may be a viable option now when the alternative is much worse.
Surrender almost always means giving up your objective anyway--often worse than dying in most adventures. You're enslaved, or imprisoned, or killed, or worse. There's a good reason most players refuse to consider surrender. If you're going to do this, you'd better make sure to communicate to your players that "surrender" does not mean all the usual horrible awful baggage that comes with it, and that it's actually possible to request a "truce" and have that not be an absolutely horrible and unacceptable choice.
To be clear in a typical 4 hours session for me there’s probably a 25% chance of a player dropping to 0 hp. What is important is that in any encounter a character might drop to 0 hp.
I find these odds extremely unlikely given my experience of 5e combat (the odds are much more like 50% even for games with few combats), but even if we take this seriously, you are then saying that you
want approximately a 1-in-24 (about 4.17%) chance of a character death
every single session. Or, to put that in slightly different terms, you want approximately 2 character deaths every year assuming weekly sessions (52 weeks with a 1/24 chance of a character death gives 2.6 average deaths per year, SD 1.57)
purely due to death by 0-hp exhaustion. This is excluding any other source of death, e.g. failing death saves.
I don’t except your premise as per my previous post. The chance of falling to 0 from a crit hit may be there, but the reality with a party of four it doesn’t happen very often. I have a group of experienced (dare I say too experienced) players they don’t (generally) make bad choices and fluff decisions. Characters are dropping to zero in 25% of sessions at most. Add in the reduced number of combats, this is a balancing mechanism to add danger where it otherwise wouldn’t exist.
See my above math. You're getting
on average 2-3 deaths a year from this. Possibly more, since there's something of a domino effect in D&D stuff--I'm just working off of your stated expectation that someone drops to 0 HP in only 25% of sessions. And, as I said, my experience with 5e combat indicates that that is wildly underestimating the actual rate.
If the party fight as a team we may find the wizard standing over the fighters prone body trying to defend him.
Your proposed rules do not give any incentive for fighting like a team. They in fact do very much the opposite. It is every man for himself, because a single bad round is enough to put you at risk of
instant death. The rules you have proposed will
punish players who make desperate last stands unless they have already accepted and embraced that their death is inevitable. That is the whole point of (as you term it) "making combat mean something." To go into combat is to court death. This creates the incentive: "Do everything you can to avoid combat, and if you engage in combat and it goes poorly, do everything you can to escape." If that is an incentive you wish your players to heed, go right ahead. It's not one I would want, but in all seriousness, my opinion of it is irrelevant; I just believe you should go into this eyes-open.